The NTP has sponsored two sodium fluoride carcinogenicity studies. Groups of F344 rats and B6C3F1 mice received 0, 25, 100 and 175 ppm sodium fluoride (NaF) in their drinking water. An equivocal tumorigenic response occurred in male rats, but no response was observed in female rats or mice. Male rats appear to be more sensitive to the potential carcinogenic effects of NaF than female rats or mice. Since irradiation can be altered to produce various osteosarcoma rates, this follow-up study would be an obvious means of evaluating the possible promoting activity of NaF. The objective is to determine whether radiation induced osteosarcoma in rats is influenced by exposure to NaF in the drinking water. Male F344 rats received 3000 rads of irradiation to the right rear limb with 137 Cesium source. Ten days later, non-irradiated and irradiated rats were given NaF at 0 and 275 ppm in drinking water daily for 2 years. The surviving rats will be evaluated histopathologically.